[Liblas-devel] Vertical Datum Transformation

[Frank Warmerdam]

Ibraheem Ali wrote:
> Dear All,
>  
> I'm a bit fresh with all LiDAR data processing and analysis.
>  
> I've got recently a bunch of LiDAR "las" format files, and their 
> Vertical datum is ellipsoidal "WGS 84". The vertical datum is not 
> defined within the las files, it was givin to me as a fact from the vendor.
>  
> I need to convert these files to NAVD88 Vertical Datum, and I basically 
> need to perform the following steps:
>  
>  1- Define the vertical datum (WGS 84) ellipsoidal within the files
> 2- Transform to NAVD 88
>  
> I've got the "liblas" with "OSGeo w4", but I can not find a clear 
> descriptions to the datums available or sample code regarding the above 
> mentioned issue
>  
> I would ask you kindly to help me with this, and would you please direct 
> me to the right documentations?

Ali,

Jason's approach with VDatum can certainly be used; however, over this
year we have put some significant effort into adding vertical datum
handling within liblas.  With OSGeo4W this should be possible, but
in addition to the liblas commandline tools package you will also need
to ensure the "proj-vdatum" package is installed from under "Libraries"
in the advanced install.

If the file already has horizontal coordinate system information based
on a geographic coordinate system using the WGS84 ellipsoid then the
source data will already be considered as using WGS84 ellipsoidal heights.

I have a source file (liblas/test/data/srs.las) which is marked as
UTM 17  / WGS84.  I can transform this to UTM 17 / WGS84 / NAVD88 with
the command:

   las2las -i srs.las -o out.las --t_srs EPSG:32617+5703

The coordinate system EPSG:32617+5703 is a shorthand way of telling
las2las (or any OGRSpatialReference based application) to create a compound
coordinate system consisting of EPSG:32617 as the horizontal coordinate
system (UTM 11 / WGS84) and EPSG:5703 as the vertical coordinate system
(NAVD88).

The las2las command will (via PROJ.4) pick up the G2003conus.gtx file
from C:\OSGeo4w\share\proj to convert from WGS84 ellipsoidal heights
to NAVD88 based heights.  The data file is also used by VDatum and
in particular for WGS84/GRS80 <--> NAVD88 transformation I have compared
the results between VDatum and this process with agreement to several
decimal places.  For the sample file the offsets are roughly 33m.

Currently the only grid based vertical datum shift files provided with
OSGeo4W are the 2003 geoid data for the USA (for NAVD88 / EPSG:5703),
and the EGM 15 global data from 1996 (for EGM96 / EPSG:5773).

Please let me know if you have any problems with this process.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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